Award Date

5-1-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

First Committee Member

Aly M. Said

Second Committee Member

Samaan G. Ladkany

Third Committee Member

Pramen Shrestha

Fourth Committee Member

Brendan J. O'Toole

Fifth Committee Member

Spencer M. Steinberg

Number of Pages

119

Abstract

Ground granulated blast furnace slag (GGBFS), a by-product from steel production, has been used as a partial replacement of portland cement in concrete for over a century. It constitutes a beneficial reuse of a by-product material, less consumption of portland cement concrete. Lower consumption of portland cement concrete can help reduce construction cost. Furthermore, it lowers the industrial carbon footprint, and landfill disposal which is a major environmental issue the world is facing today. The chemical composition of GGBF slag is similar to portland cement with hydraulic properties and additional pozzolanic properties with excellent alkali silica reactivity mitigation and resistance to chemical sulfate attack. The major limitation of slag are its slower early strength gain, susceptibility to surface scaling where concrete slabs is exposed to freezing and thawing in the presence of moisture and deicing salts. Concrete containing pozzolans also requires less cement content to reach its design strength. Pozzolanic reaction is generally slow compared to hydration reaction, which limits the cement replacement by pozzolans. Nano-silica can accelerate the pozzanic reaction due to its high surface area to volume ratio. This characteristic of nano-silica also promotes the cement hydration reaction between produces C–S–H gel and Ca(OH)2. Calcium hydroxide is then consumed by the pozzlanic reaction with nano-silica and GGBF slag and produces more binding C–S–H gel in the system. The hypothesis of this study is that the use of ternary blends of portland cement, GGBF slag and nano-silica, could allow the effect of one study material to compensate for the inherent shortcomings of another. The aim of this research is to investigate the properties of concrete containing ordinary portland cement (OPC) replaced by 50% GGBF slag with two different ratios of nano-silica (3% and 6% of the total cementitous material). In this regard, reactivity, mechanical properties and the durability of the studied mixtures were tested. Furthermore, when slag is used for alkali-silica reactivity (ASR) mitigation, a 50% slag cement is is typically recommended. However, such high cement replacement by slag is associated with salt scaling issues. Also, scarcity of high quality (120 grade) GGBF slag in some markets may limit its use on large scale. Another goal of the current study is to mitigate ASR problem using a lower percentage of cement replacement with slag. Additionally, the physical salt attack (PSA) on concrete, a phenomenon that is sometimes misidentified as a chemical salt attack, may cause significant damage to concrete cast in contact with sulfate rich soils in some climates.. The physical effect of salt was explored by combining the quantitative X-ray diffraction(XRD) with the Rietveld refinement method. Currently, no standard test and no code provision are available for PSA, so to investigate this phenomenon’s effect on the studied mixtures, an environmental chamber was used to simulate seasonal changes in the regions where concrete is exposed to this type of distress with the specimens partially immersed in the high concentration salt solution. To further verify the observed physical and mechanical changes imparted to the concrete with slag through the addition of nano-silica, microstructure of the cement matrix. Accordingly, the tested mixtures were further examined by mercury intrusion porosimetry (MIP), energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM).

Keywords

EDX; MIP; Nano-silica; Nanosilicon; Pozzuolanas; SEM; Slag; Slag cement; Steel Portland cement – Mechanical properties; XRD

Disciplines

Civil Engineering | Engineering | Materials Science and Engineering

Language

English

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